diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index e132c65..32540b4 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -7011,6 +7011,44 @@ public sealed class GameWindow : IDisposable
             var visibility = _cellVisibility.ComputeVisibility(camPos);
             bool cameraInsideCell = visibility?.CameraCell is not null;
 
+            // SPIKE 2026-05-26: A8 transition investigation. Lights up the
+            // dormant RenderingDiagnostics.ProbeVisibilityEnabled flag (added
+            // by Task 6 of the original A8 plan). Per-frame state captures:
+            // camera position, lenient + strict inside flags side-by-side,
+            // CameraCell id, VisibleCellIds list. Branch markers inside
+            // indoor + outdoor branches complete the trace.
+            // Enable via ACDREAM_PROBE_VIS=1.
+            if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeVisibilityEnabled)
+            {
+                bool reallyInsideProbe = visibility?.CameraCell is not null
+                    && CellVisibility.PointInCell(camPos, visibility.CameraCell);
+                int visCount = visibility?.VisibleCellIds?.Count ?? 0;
+                string visList;
+                if (visibility?.VisibleCellIds is null || visCount == 0)
+                {
+                    visList = "[]";
+                }
+                else
+                {
+                    var sb = new System.Text.StringBuilder("[");
+                    int shown = 0;
+                    foreach (var id in visibility.VisibleCellIds)
+                    {
+                        if (shown >= 8) { sb.Append(",..."); break; }
+                        if (shown > 0) sb.Append(',');
+                        sb.Append($"0x{id:X8}");
+                        shown++;
+                    }
+                    sb.Append(']');
+                    visList = sb.ToString();
+                }
+                string cellId = visibility?.CameraCell?.CellId.ToString("X8") ?? "null";
+                Console.WriteLine(
+                    $"[vis] pos=({camPos.X:F2},{camPos.Y:F2},{camPos.Z:F2}) " +
+                    $"inside={cameraInsideCell} really={reallyInsideProbe} " +
+                    $"cell=0x{cellId} visN={visCount} {visList}");
+            }
+
             // Lighting decisions (sun zeroed, indoor ambient applied) must
             // track the PLAYER's cell, not the camera's. In third-person
             // chase mode the camera enters interiors before the player body
@@ -7174,6 +7212,8 @@ public sealed class GameWindow : IDisposable
             if (cameraReallyInside && _indoorStencilPipeline is not null
                 && visibility?.CameraCell is not null)
             {
+                if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeVisibilityEnabled)
+                    Console.WriteLine("[vis] branch=indoor");
                 // Phase A8 R3 — WB RenderInsideOut order.
                 //
                 // 1. Terrain has already drawn (color + depth) at line ~7104.
@@ -7233,6 +7273,8 @@ public sealed class GameWindow : IDisposable
             }
             else
             {
+                if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeVisibilityEnabled)
+                    Console.WriteLine("[vis] branch=outdoor");
                 // Outdoor path — unchanged from pre-A8: single dispatcher call
                 // walks every entity with default EntitySet.All partition.
                 _wbDrawDispatcher!.Draw(camera, _worldState.LandblockEntries, frustum,